The major function of koji during fermentation is to supply various enzyme sources for digesting various components of rice, wheat/barley and soybean. Also, the koji provides nutrients for promoting the propagation, growth and fermentation of valuable microorganisms and imparts flavor to the products of koji-mold and to the fermented products of koji-mold mass.
Accordingly, the object of koji-making is to properly form and build up enzymes, to promote the growth of valuable microorganisms by the growth of koji-mold and to form a flavor component precursor. Thus, koji-making is carried out for the purpose of obtaining koji products having an appearance and flavor suitable as koji.
Currently available koji-making methods have been developed on the basis of the quality of koji products which have been obtained by the koji-tray method which have been attained by the result of trial and error. According to the koji-tray method, koji is made by introducing cooked rice beneath a floor for seeding, fermenting it for about 20 hours and then placing it on a koji-tray to a thickness of about 4 to 5 cm. During the process, the temperature of the fermenting material is controlled by suitable procedures such as during the first, second and last treatments in koji-making. The temperature of the fermenting material is controlled because it rises sharply during the growth of koji-mold. The humidity is also regulated so as to set up suitable conditions under which the koji-mold grows easily.
The koji-tray method is best suited to impart flavor to fermented foodstuffs because the resulting koji excels in appearance, possesses a peculiar flavor, and is soft to the touch. However, the koji-tray method must be carried out under koji-making conditions which are very detrimental to the system in view of physiology and it requires a great deal of labor.
In order to eliminate the defects of the above-mentioned koji-tray method, the mechanization of koji-making has been expedited primarily to save labor. However, it has been found that the quality of the resulting koji is still unsatisfactory.
In recent years, a number of new techniques have been developed with regards to the mechanization of koji-making so as to obtain koji products which are equivalent in quality to those obtained by the koji-tray method. According to the mechanized arrangement, koji is built up to a considerable thickness and in a large amount in a vessel of large dimensions. This arrangement also provides for the forced aeration during the koji-making. Air which has been regulated to a suitable temperature and humidity is forced through a koji-layer. The koji-layer is turned upside down by mechanical means, such as with rotary stirring, at the time when heat is so vigorously generated during the propagation or growth of the koji-mold so that koji-strains grow and entangle with each other. This agitation results in a koji-layer which is disentangled in order to promote aeration and so that the temperature and humidity of the fermenting material can be suitably regulated.
However, it has been found that forced aeration carried out by rotary stirring adversely affects the quality of koji because it considerably cuts the koji-strains as a result of the treatment by hand which involves the reversal of the koji-layer. Moreover, this forced aeration by rotary stirring results in lumps of koji which may result in over-heating as well as a delay in the growth of strains after treatment by hand.
Furthermore, the growth of koji-mold becomes so vigorous during aeration type koji-making at later stages that heat is vigorously generated and the dissipation of moisture reaches a maximum. This leads to further growth and entanglement of koji-strains, which in turn results in irregular and local cracks as well as local aeration. When koji portions which are undergoing unsatisfactory aeration are permitted to stand over an extended period of time in a state wherein the temperature of the koji generates abnormal amounts of heat which is locally and excessively increased, the flavor of koji not only deteriorates, but proper growth of koji-mold is also affected, thus leading to an unsatisfactory saccharification.
To avoid these disadvantages, the treatment by hand has been repeatedly carried out by mechanical operations such as by stirring. It has been found, however, that the strains are so increasingly cut that the proper growth of mold is increasingly inhibited. For the same purpose, it has been proposed to increase the amount of aeration and to lower the temperature. However, it has been noted that difficulty is encountered in obtaining koji of good flavor, since koji is excessively contacted by a large amount of cold air, and the mold mass which is produced is exposed to a large amount of air over an extended period of time at a stage where the formation and accumulation of flavor reach a maximum.